Paper: Loh, T-L, S. E. McMurray, T. P. Henkel, J. Vicente, and J. R. Pawlik. 2015. Indirect effects of overfishing on Caribbean reefs: sponges overgrow reef-building corals. PeerJ 3eo901; DOI: 10.7717/peerj.901.
Note: If this topic sounds familiar, this study was briefly covered as a highlight of the 2015 Benthic Ecology Meeting. We have more details here!
If someone were to tell you that sponges are taking over, you wouldn’t exactly be left quaking in your boots. It’s a sponge! It’s harmless! Well, to a coral, a sponge is a competitor and a tough one at that! In a world with so many threats to valuable coral reefs, sponges don’t come to the forefront of people’s minds. However, one threat that does is overfishing, which may influence sponge abundance and coral-sponge interactions.
In this study, the research team sought to demonstrate that the prevalence of sponge-coral competition is an indirect effect of human fishing activity on Caribbean reefs. Throughout the Caribbean, humans have been running what the authors describe as a “long-term manipulative experiment” for decades. In this experiment, people have chosen to heavily fish certain reefs while leaving others relatively protected (such as Marine Protected Areas or regions with low human population densities). Now, the research team of Loh et al. has decided to look at the effects of that experiment!
What did the scientists do?
The research team surveyed 69 sites in 12 countries in the Caribbean with either extreme fishing intensity (overfished) or virtually no fishing at all (less-fished). They counted spongivorous fishes (or fish species that eat sponges, e.g. parrotfish and angelfish (Figs. 2)) and calculated an index of grazing activity (the Spongive Index or SI). The researchers also surveyed the benthic community to see how much of the bottom was covered with corals, sponges, sea fans, algae, etc. Finally, they categorized coral colonies by their level of interaction with sponges: having no contact with sponges, adjacent to and in contact with sponges, or overgrown by sponges (Fig. 3, below).
What They Found:
First, they found that grazing activity by spongivores was much lower at overfished sites (SI of ~2 per 2000 m3) than less-fished sites (SI of ~43 per 2000 m3). Only 12% of coral colonies at less-fished sites had interactions with sponges (8.8% adjacent to and 3.2% overgrown by them), but 25.6% of the colonies at overfished sites were interacting with sponges (14.9% adjacent to and 10.7% overgrown by them) (Fig. 4). That means that twice the number of corals on overfished reefs are actively competing with sponges! These findings by themselves show that indeed overfishing does indirectly affect the competition between corals and sponges.
They also found that at less-fished reefs (where there are lots of sponge-eaters), most sponges that are overgrowing corals (70.9%) are slow-growing species with chemical-defenses, while most (43%) on overfished reefs are the palatable, fast-growing species. It seems then that overfishing also affects the competition among sponges themselves.
Finally, they noticed something very interesting about another component of the benthos. Macroalgal cover was higher on less-fished reefs (38.1%) than on overfished ones (23.1%). The prevailing understanding is that where there are more herbivorous fish, there should be less macroalgae, not more! The authors suggest this may be due to increased nutrients from fish excrement, absence of the historically dominant herbivorous sea urchin Diadema antillarum, or differences in the macroalgae species (such as palatability) at the different sites. They even suggest that overfishing may remove sea urchin predators, thereby allowing sea urchins to rebound more rapidly and control macroalgae better on overfished reefs. They encourage further studies to try to explain this result more completely.
Why It’s Important:
Again, sponges don’t generally get a lot of press (unless it’s Spongebob Squarepants!), but we should care about them as part of the coral reef ecosystem. When seven species of Caribbean corals are listed as threatened under the Endangered Species Act (Learn more), it is important to pay attention to all the factors driving their declines and to make sure we don’t overlook indirect effects. The authors suggest the results of their study justify imposing fishing restrictions that would protect parrotfish and angelfish to minimize sponge-coral competition, especially given that sponges have been documented in other works as the superior competitor. They defeat their coral opponents by shading, smothering, and chemically attacking coral tissue, all of which can cause the coral to bleach, become susceptible to disease, and eventually die. Overfishing disrupts the balance between sponges and corals by removing fish that help check sponges’ competitive advantage and thereby threatens vulnerable corals!
One other take-away message from this research is the importance of location! While it’s easy to make generalizations about how coral reef ecosystems function around the world, there is a growing body of evidence that Caribbean reefs differ from those of the Indo-Pacific in both the role and abundance of sponges as well as the ability of herbivorous fish to control the abundance of macroalgae through multi-level ecosystem interactions.
In summary, these researchers showed that angelfish and parrotfish consumers exert control over not only the abundance of but also the competition among sponges and corals by demonstrating how human fishing activity is influencing that system. And the most beautiful part, they used basic ecological concepts to do it!
Feedback: Now that you know sponges are an important component of coral reef communities, are you more likely to pay attention to them? Besides fishing regulations, are there other types of management you’d recommend to protect important sponge-eaters?